1. Field of the Invention
The present invention relates generally to ATM (asynchronous transfer mode) switching technology, and more specifically to system expansion using ATM switching modules of different types without losing cells during cutover.
2. Description of the Related Art
ATM switches have been developed for high speed network applications such as broadband ISDN. For ease of system expansion, an ATM switch is usually constructed of Nxc3x97N configuration to be used as a basic building block to implement a switching system of desired size. To ensure non-blocking operation during system expansion, N/2 ports of the switch are preassigned to expansion switching modules. Since the preassigned ports are normally reserved as idle ports, utilization efficiency of the input ports of a switch is low for applications if the number of subscribers is smaller than N2. This is particularly true when the operating speed of subscriber is low. In some cases, subscriber""s per-port usage is much lower than the processing capacity of the switch. if this fact is taken into account with the link capacity of expansion interfaces, non-blocking operation can still be realized with switches whose preassigned ports for expansion are smaller than N/2, particularly in switching nodes at the edge of a public switched telephone network. This prior art technique is described in Japanese Laid-Open Patent Specifications Hei-4-273739 and Hei-7-115421.
In addition, two approaches are currently available for system expansion, one involving the use of a number of identical basic switching modules, while the other using a large-sized switching module as an intermediate stage. In these techniques, precautions are taken against losing cells during cutover of the expansion units. However, it is impossible to incorporate the large-sized switching module in an expansion process in which the basic switching modules are exclusively used without causing cell loss, since the fixed number of preassigned ports allows no freedom to flexibility in combining different types of switching modules.
It is therefore an object of the present invention to provide an ATM switching module that can be used with a different type of switching module during system expansion without causing cell loss.
According to a first aspect of the present invention, there is provided an ATM switching module comprising a plurality of line interfaces, each line interface having a routing table for receiving an ATM cell, making a search through the routing table for routing data according to a connection identifier contained in the cell, and updating header of the cell according to the routing data. An ATM switch is provided for routing the ATM cell according to the updated header. A plurality of pairs of expansion interfaces are provided such that the expansion interfaces of each pair being connected together to the ATM switch to operate in active and standby states, respectively. A processor is responsive to another ATM switching module being installed and a connection being established between one of the expansion interfaces of standby state and the Installed ATM switching module, for setting the standby state expansion interface in hot-standby state, holding off incoming ATM cells destined for the hot-standby state expansion interface until the connection is ready to accept cell traffic, and switching the hot-standby state expansion interface to active state, whereby the ATM switch and an ATM switch of the newly installed switching module constitute a multi-stage configuration. The processor is further arranged to update the routing table according to internal state of the ATM switch so that, when a call is to be established for an incoming ATM cell, the header of the cell contains a plurality of fields containing routing data respectively associated with different stages of the multi-stage configuration.
According to a second aspect, the present invention provides a method for expanding a communications network, wherein the network comprises a plurality of ATM switching modules, each of the modules comprising a plurality of line interfaces, each line interface having a routing table for receiving an ATM cell, making a search through the routing table for routing data according to a connection identifier contained in the cell, and updating header of the cell according to the routing data, an ATM switch for routing the ATM cell according to the updated header, a plurality of pairs of expansion interfaces, the expansion interfaces of each pair being connected together to the ATM switch and operating in active and standby states, respectively. The method comprises the steps of (a) Installing an ATM switching module as an additional unit, (b) establishing a connection between one of the expansion interfaces of standby state and the installed ATM switching module, (c) setting the standby state expansion interface to hot-standby state and holding off incoming ATM cells destined for the hot-standby state expansion interface until the connection is ready to accept cell traffic, (d) switching the hot-standby state expansion interface to active state, whereby the ATM switch and an ATM switch of the another switching module constitute a multi-stage configuration; and (e) responsive to an incoming ATM cell, updating the routing table according to internal state of said ATM switch so that the header of the cell contains a plurality of fields containing routing data respectively associated with different stages of the multi-stage configuration.